This invention relates to an offshore apparatus for drilling, production, and storage of oil at a deep water well site and to a method of fabrication, transport in towing mode and draft, and of a method of transition from towing draft to operating draft. The construction and method involve upper and lower hulls arranged in telescopic relation for towing and in vertically extended nontelescopic relation for operations at a well site.
Prior proposed floating vessels for drilling, production, and storage of oil at sea have included semisubmersibles, spars or caissons of great length, and semispars. A deep draft spar of great length is shown and described in my U.S. Pat. No. 4,702,321. Shallow draft stepped spars such as FLIP and the Brent spar, as well as the deep draft spar,require transit to a well site in horizontal position and then are ballasted to a vertical position to an operating draft. The Flip spar is a floating instrument platform including a long (about 350 feet) cylinder necked down at the water line. The Brent spar is a long (about 300 feet) cylindrical spar for storing oil. In some instances, decks are installed in upright position because the size of the deck precludes horizontal towing. Other step spars have been designed for towing in vertical position. One such step spar was constructed of concrete using slip forms in vertical arrangement and towed in vertical position. Towing in vertical position resulted in designs having restricted transit routes, limited draft, such as 120 feet, and presented problems of stability during towing. Usually a single hull construction was involved.
Prior proposed offshore structures have included floatable barges supporting upstanding columns and means including decks movable vertically with respect thereto into position above the sea surface. In many instances the floatable barge was sunk to the sea floor and served as a base. Heave motion of the structure at the well site was not a design consideration.
British specification 991,247 published May 5, 1965 shows a structure adaptable for semisubmersible operation in which a floatable bottom grid supports columns along which a deck is vertically movable, the columns being laterally reinforced by bracing.
UK Patent Application GB 2,003,964 shows a method of mounting a deck on a marine structure comprising a submerged caisson with an upstanding tower over which the deck may be floated, the caisson being then raised to move the tower relative to the deck, and the deck then secured to the tower.
Such prior proposed structures did not contemplate a spar construction having a deep draft in operating position to achieve low heave motion and a shallow draft for stable transit to a well site.